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This invention relates generally to magnetic resonance imaging (MRI), and more particularly, the invention relates to flow imaging using a phase contrast Steady State Free Precession MRI sequence.
Magnetic resonance imaging (MRI) requires placing an object to be imaged in a static magnetic field, exciting nuclear spins in the object within the magnetic field, and then detecting signals emitted by the excited spins as they precess within the magnetic field. Through the use of magnetic gradient and phase encoding of the excited magnetization, detected signals can be spatially localized in three dimensions.
FIG. 10A is a perspective view partially in section illustrating conventional coil apparatus in an NMR imaging system, and FIGS. 10B-10D illustrate field gradients which can be produced in the apparatus of FIG. 10A. This apparatus is discussed by Hinshaw and Lent xe2x80x9cAn Introduction to NMR Imaging: From the Block Equation to the Imaging Equation.xe2x80x9d Proceedings of the IEEE, Vol. 71, No. 3, March 1983, pp. 338-350. Briefly, the uniform static field B0 is generated by the magnet comprising the coil pair 10. A gradient field G(x) is generated by a complex gradient coil set which can be wound on the cylinder 12. An rf field B1 is generated by a saddle coil 14. A patient undergoing imaging would be positioned within the saddle coil 14.
In FIG. 10B an X gradient field is shown which is parallel to the static field B0 and varies linearly with distance along the X axis but ideally does not vary with distance along the Y or Z axes. FIGS. 10C and 10D are similar representation of the Y gradient and Z gradient fields, respectively.
FIG. 11 is a functional block diagram of conventional imaging apparatus as disclosed in NMR-A Perspective in Imaging, General Electric company. A computer 20 is programmed to control the operation of the NMR apparatus and process FID signals detected therefrom. The gradient field is energized by a gradient amplifier 22, and the rf coils 26 for impressing an rf magnetic moment at the Larmor frequency are controlled by the transmitter 24. After the selected nuclei have been flipped, the rf coil 26 is employed to detect the FID signal which is passed to the receiver 28 and thence through digitizer 30 for processing by computer 20.
Investigation of blood flow in the heart and vessels can provide insight in the function of the cardiovascular system. Magnetic resonance imaging with multidirectional CINE velocity mapping can be used to study relationships between aortic and left ventricular blood flow patterns and the geometry of the thoracic aortic aneurysms and grafts. Recognizable altered flow patterns were found to be associated with altered vessel geometry. The results of velocity mapping of aortic wall motion as well as pulse wave velocities can be combined with distensibility and stiffness index in order to find potential correlations of these parameters.
Fully balanced Steady State Free Precession (SSFP) imaging has recently gained increased importance due to its high signal-to-noise ratio (SNR). The gradient waveforms have zero net area in each repetition time (TR) interval and are first order moment nulled along read and slice direction at each rf excitation.
The inventors recognized, however, that during data acquisition, SSFP sequences are motion sensitive and the MR-signal exhibits a motion related phase depending on the first moments of the gradient activity between rf-excitation and data readout. Flow or motion quantification can thus be accomplished but, as in conventional phase contrast (PC) MRI, a method to remove other phase shifts (e.g. due to off-resonance) is needed. In accordance with the invention, a novel technique for velocity measurements (PC-SSFP) combines CINE phase contrast (PC) MRI and balanced Steady State Free Precession. Sensitivity to through plane velocities is established by inverting (i.e. negating) the slice select gradient for consecutively executed balanced SSFP pulse sequences. Velocity sensitivity (venc) can be adjusted by increasing the first moments of the slice select gradients. Comparison of measurements on phantoms with those from established 2D CINE MRI demonstrated excellent correlation between both modalities. Advantages of PC-SSFP include the intrinsic high signal to noise ratio of balanced SSFP and consequently low phase noise in encoded velocities.
Velocity encoding in the slice direction is performed by data acquisition with two SSFP sequences which differ only in the sign of all gradients along the slice selection direction. First moments M1+ and M1xe2x88x92 associated with the slice selection gradients are thus altered accordingly. As a result phase difference images xcex94xcfx86 can be calculated which are directly related to the velocity v2. Similarly, velocity in the readout direction can be encoded by performing two sequences which differ only in the sign of all the gradients along the readout direction. Velocity effects in the phase encoded direction can be achieved by applying a bipolar lobe in this direction prior to readout whose effect is undone by a bipolar lobe after readout. Velocity encoding in the phase encoded direction is achieved by data acquisition with two SSFP sequences which differ only in the amplitude or sign of the bipolar lobes.
Different velocity sensitivities in the slice select direction can be realized by lengthening the plateau of the slice selection gradient and adjusting refocusing and prefocussing gradients accordingly while keeping the rf-pulse width constant and thus keeping TR at a minimum, or by changing the rf-bandwidth and gradient strength. Similarly, different velocity sensitivities in the readout direction can be achieved by controlling the length and amplitude of the gradients in this direction in conjunction with appropriate control of the readout bandwidth. Different velocity sensitivities in the phase encoded direction are achieved in a straight forward manner by control of the bipolar lobes.
The invention and objects and features thereof will more readily apparent from the following description and appended claims when taken with the drawings.